1. Field of the Invention
The present invention relates to a communication terminal, a communication system, a congestion control method, and a congestion control program, and particular, a communication terminal, a communication system, a congestion control method, and a congestion control program which perform low-delay communication by using a broadband line.
This application is based upon and claims the benefit of priority from Japanese patent application No. 2007-024525, filed on Feb. 2, 2007, the disclosure of which is incorporated herein in its entirety by reference.
2. Related Art
In communication through a network, in order to low-delay communication between transmission and reception terminals, delay time by a queueing delay in the network is required to be shortened. A minimum value of delay between the transmission and reception terminals is propagation delay time between the transmission and reception terminals, and communication with delay shorter than the propagation delay time may not be able to be performed. When congestion occurs in the network, a queue to wait for a packet output is generated in a node where the congestion occurs, and delay between the transmission and reception terminals is increased by time for which the packet passes through the queue. When the queue becomes long over a buffer capacity of the congested node, packet discard is generated. For this reason, the delay time between the transmission and reception terminals considerably increases by time required for packet retransmission. In this manner, in order to perform low-delay communication, a queueing delay in the node in the network must be reduced.
The most effective method serving as a method for reducing a queueing delay is a method which stores only low-delay communication in a queue having an especially high priority in the in-network node to preferentially output a packet. For example, a method achieved by Differentiated Service (Internet Engineering Task Force, Request For Comments 2475) and the like are given. However, in this method, a special device in the in-network node is required, a queueing delay still occurs when a plurality of low-delay communications are simultaneously performed.
As another method, a method in which an in-network node explicitly notifies a transmission terminal of congestion information may be used. As this method, an ECN (Explicit Congestion Notification, Internet Engineering Task Force, Request For Comments 3168) method in which a degree of congestion of a node is written in a packet header to notify a transmission terminal of the packet or a method in which an optimum transmission bandwidth of each communication is calculated and is communicated to the transmission terminal as described in “Processor Sharing Flows in the Internet” (Nandita Dukkipati, Masayoshi Kobayashi, Rui Zhang-Shen, Nick MacKeown, Thirteenth International Workshop on quality of Service (IWQoS)), and the like are known. However, these methods require special devices in in-network nodes, and cost for switching existing nodes is necessary.
As techniques different from the above methods, the following techniques for realizing low-delay communication by congestion control between transmission and reception terminals are known.
In the first related art is a technique which sets a maximum value of a transmission bandwidth depending on a bandwidth of a network. In this technique, an excessive transmission bandwidth over the bandwidth is prevented from being set, so that an unnecessary queueing delay or packet waste can be prevented. For example, in the method described in Japanese Patent Application Laid-Open (JP-A) No. 2006-279283, an optimum communication bandwidth is estimated during TCP communication, and congestion control is performed such that a transmission bandwidth does not exceed the communication bandwidth. In this manner, unnecessary congestion is prevented to make data transmission delay time short.
The second related art is a technique which determines a transmission bandwidth on the basis of one-way delay time or round-trip delay time measured by a transmission terminal. In this technique, when delay time increases, by reducing a transmission bandwidth, congestion control is performed in the transmission terminal to keep a queueing delay short. For example, in TCP-FAST method (From Theory to Experiments, C. Jin, D. X. Wei, S. H. Low, G. Buhrmaster, J. Bunn, D. H. Choe, R. L. A. Cottrell, J. C. Doyle, W. Feng, 0. Martin, H. Newman, F. Paganini, S. Ravot, S. Singh. IEEE network, 19(1): 4-11, Jan./Feb. 2005), round-trip propagation delay time of a network is measured, by using a value (corresponding to a data amount in a corresponding communication which is caused to wait by a queue in a node) obtained by multiplying a difference between the minimum value of the round-trip propagation delay time and a present value by a present transmission bandwidth, the transmission bandwidth is controlled such that the value is induced to be a predetermined target value, so that the transmission bandwidth is controlled to an optimum bandwidth.
As techniques related to the present invention, JP-A No. 2000-295286, JP-A No. 2004-153776, JP-A No. 2006-340078, and JP-A No. 2005-517330 are known.
A disadvantage of the first related art is that when a plurality of sessions are present, even though a limited bandwidth of each of the sessions is a bandwidth or less, a total of limited bandwidths of these sessions exceeds the bandwidth. For this reason, when a plurality of sessions are present, a queueing delay becomes long.
A disadvantage of the second related art is that a packet having a set data amount is stationarily accumulated, and the number of accumulated packets varies to considerably vary delay time. In order to perform low-delay communication, a transmission bandwidth must be early achieved to an optimum transmission. For this purpose, an increase range of the transmission bandwidth must be set to be large. However, when the increase range of the transmission bandwidth is set, a large number of packets reach a network node for a short period of time. For this reason, a queue length considerably increases within a short period of time. However, time taken until congestion is communicated to a transmission terminal becomes long due to the increase in queueing delay. As a result, when a transmission bandwidth is sharply increased, time taken until the transmission bandwidth is reduced accordingly after the congestion occurs becomes long. As a result, the queue length becomes unstable. Furthermore, in the first related art, the number of packets stationarily accumulated in a queue disadvantageously linearly increases depending on an increase in number of sessions.